(1.) Field of the Invention
The present invention relates to an optical disk apparatus.
(2.) Description of Related Art
In Japanese Unexamined Patent Application Publication No. 2002-342952, a recording & reproducing apparatus of optical recording media equipped with control units of a focus servo and a tracking servo for aberration correction is disclosed. In addition, in Japanese Unexamined Patent Application Publication No. 2005-332558, an optical disk apparatus equipped with a spherical aberration correcting section that corrects the spherical aberrations generated in optical beams in advance, a focus error detecting unit that detects focus error signals, and a control unit that adjusts the signal amplitudes output from the focus error detecting unit to predefined values after the amounts of spherical aberrations are made equal to predefined amounts by the spherical aberration control unit.
There are cases where the recording surfaces of optical disks on which an optical disk apparatus records data and from which the optical disk apparatus reproduces data have so called birefringent portions. The birefringent portions come into existence owing to internal stresses that are generated during the disk production process. If there are birefringent portions on the recording surface of an optical disk, when laser beams are focused on the recording surface, the patterns of the focused light spots get distorted or blurred so that the amounts of the reflected lights may decrease or improperly change. Such a decrease or change of the amounts of the reflected lights deteriorate reproduced signals and servo signals that are generated with the use of the reflection lights, with the result that the so-called servo control for putting the focused light spots on the tracks of the recording surfaces will get unstable.
In either of the above-mentioned patent documents, however, there are no measures taken to prevent the deteriorations in the quality of reproduced signals or servo signals owing birefringence.